Fixing of electrophoretic deposits

ABSTRACT

IN FIXING BY EXPOSURE TO RADIANT HEAT AN ELECTROPHORETICALLY PRODUCED DEPOSIT UPON AN ANODIZED SURFACE, THE PROCESS IS EXPEDITED (OR, ALTERNATIVELY, THE REQUIRED TEMPERATURE IS REDUCED) BY DEPOSITING UPON THE SAID DEPOSIT, PRIOR TO HEATING, AN AUXILIARY BLACK DEPOSIT OF CARBON WHICH HAS VERY HIGH ABSORPTIVITY AND SO RECEIVES THE RADIANT HEAT FASTER THAN THE NON-BLACK DEPOSIT. THE AUXILIARY COATING IS REMOVED AFTER COMPLETION OF THE HEATING OPERATION.

United States Patent O U.S. Cl. 204-181 1 Claim ABSTRACT OF THE DISCLOSURE In fixing by exposure to radiant heat an electrophoretically produced deposit upon an anodized surface, the process is expedited (or, alternatively, the required temperature is reduced) by depositing upon the said deposit, prior to heating, an auxiliary black deposit of carbon which has very high absorptivity and so receives the radiant heat faster than the no'n-black deposit. The auxiliary coating is removed after completion of the heating operation.

REFERENCE TO RELATED APPLICATION S.N. 713,949 filed Mar. 18, 1968, by Irving J. Hess for Self-Cleaning Electrodeposits.

BACKGROUND OF THE INVENTION Field of the invention This invention pertains to the art of coating, in particular to the art of coating by electrophoresis.

Description of the prior art In my copending application of reference it is explained how to apply a coating of finely suspended particles by electrophoresis to an anodized substrate, and to fix the coating of such particles by heating the substrate with the coating thus deposited. Such heating is most conveniently affected by exposing the workpiece to radiant heat, as by immersing it in a heated muffle or tube furnace. Since the absorptivity of the particles of the coating will determine the rate at which radiant heat will be absorbed and consequently the time required to affect the fixing, that time will vary depending upon the nature of the coating, being in general longer for a highly reflective, slightly absorbing, coating than for a dark one otherwise of the same characteristics. This fact tends to prolong the heat fixing of light coatings; and the variation in processing time which it introduces is a complication in any automatic process for producing such coatings.

SUMMARY OF THE INVENTION An anodized substrate (workpiece) is coated by electrophoresis with a desired permanent coating. The desired permanent coating is then coated, in a different bath, with a thin layer of carbon. The doubly coated substrate is then exposed to radiant heat to fix the desired permanent coating to the workpiece, the required time being minimized, (or the required temperature being reduced) and rendered independent of the absorptivity of the desired by the presence of the external carbon coating. It has been found that the auxiliary superficial deposit of carbon may then be readily removed by a jet of lowpressure air or low-velocity water.

Thus I achieve the desirable object of heat fixing the coated pigment deposit, regardless of the reflectivity of the pigment, in a minimal time at a given temperature, or a minimal temperature for a given time, and rendering these time and temperature requirements uniform for pigments of differing reflectivity. The attainment of these objects achieves economy of operation, and facilitates control of time and temperature.

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'2 BRIEF DESCRIPTION OF THE DRAWING Since this invention pertains only to a method for applying a coating, in which the novelty does not depend upon any structural features, its nature does not permit of illustration by a drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Panels of an aluminum alloy known commercially as 6061 (nominal composition in percent by weight: copper 0.25, silicon 0.6, magnesium 1.0, chromium 0.25, remainder aluminum and casual impurities), 0.040 inch thick and 1 by 5 inches, were anodized for 15 minutes at a current density of 40 amperes per square foot at 25 degrees C. in a bath of 25 percent by weight of sulphuric acid in water. They were rinsed in water to remove the bath traces, and dried in air. They were then immersed in a suspension of 10 grams per liter, in methyl-ethyl ketone, of an organic pigment known commercially as Harmon R6502 Indofast Brilliant Scarlet press cake, typical of commercially available pigments, sold by the National Aniline Division of Allied Chemical Company. A potential of 500 volts was applied for 10 seconds between the workpieces and an inert electrode immersed in the bath, to cause electrophoretic deposition of the pigment upon the anodized surfaces of the panels, in conventional fashion. Half of the panels were then immersed in a dispersion of 10 grams per liter of carbon black in isopropyl alcohol. Carbon black was deposited upon these by application of a potential of 100 volts for 5 seconds. The workpieces were all dried in a gentle coating of air, and then fixed by immersion in a muflle furnace as indicated in the following table:

Temperature, "/C. Time, seconds 450 90, 60, 40, 30 400 90, 60, 40, 30

In each of the indicated cases, the carbon-overcoated sample was blown with a jet of air to remove the carbon coating so that the pigment beneath was exposed, and compared with the otherwise identically treated sample which had been heated without a carbon coating. In all cases, the color of the pigment carbon-overcoated during heat treatment was darker than the identically treated but not carbon overcoated sample. Specifically, a panel heated without carbon overcoat at 450 degrees for 60 seconds was comparable in color with a panel heated with carbon overcoat at 400 degrees for 60 seconds. Since the pigment tends to be darkened by the heat treatment, it is evident that the application of carbon overcoating does produce the equivalent of higher-temperature heat treatment for the same period of time without carbon overcoating. Similarly prepared samples showed the same coating color for a panel heat treated without carbon overcoating for seconds at 450 degrees, and one heat treated with carbon overcoating for 70 seconds at 450 degrees. The adherence of the pigment coating was equally good in all cases.

It is thus evident that the superficial overcoating of carbon does produce a superior absorption of radiant energy which produces in the desired permanent pigment coating beneath the same effect as is produced in a pigment coating without carbon overcoating only by the use of higher furnace temperature, or a longer heat treating or fixing time.

Since the carbon overcoating is not required to perform any function other than absorption of radiation, the conditions of applying it electrophoretically are not critical. I have employed successfully voltages from 25 to 1000 volts, and deposition times ranging from 1 to 15 seconds.

What is required is simply that the black overcoating be substantially complete so that it covers the desired pigment coating, although even an incomplete coating (which there is no apparent reason to desire or use) would be expected to produce some improvement in absorption of radiation. Since the effect is merely one of heating and not chemical reaction (as is evidenced by the ready removal of the carbon after the heating operation) the particular nature of the desired pigment is immaterial, provided only that it may be fixed by heating.

What is claimed is:

1. In the process of applying a coating of pigment to an anodized surface by:

(a) Depositing a coating of the pigment electrophoretically from a liquid suspension upon the anodized surface;

,(b) Drying the anodized surface bearing the said coating of pigment;

(0) Heating the said coating of pigment upon the said anodized surface by exposure to radiant heat to fix it thereupon, the improvement comprising:

(d) Depositing upon the said coating of pigment, prior to the said heating step (c), an overcoating of carbon 5 black by electrophoresis;

(e) Performing the said heating step with the said carbon overcoating upon the said coating of pigment; and

10 (f) Removing the said overcoating of carbon black after the completion of the said heating step (c) to expose the said coating of pigment.

References Cited UNITED STATES PATENTS 3,493,482 2/1970 Irwin 204-181 3,362,899 1/1968 Gilchrist 204-48] JOHN H. MACK, Primary Examiner 20 R. L. ANDREWS, Assistant Examiner 

